In the asymmetric Mach-Zehnder interferometer optical splitter circuit, a light beam is split and the split light beams are input to each of two arm waveguides, and the light beams from the respective arm waveguides are combined, then split, and then then output to two output waveguides. An optical splitting ratio, which is a ratio of the intensity of the light beams output to the respective output waveguides, changes according to a phase difference between the light beams from the respective arm waveguides. Thus, in order to obtain a desired optical splitting ratio, the phase difference between the light beams needs to be set appropriately.
As a method for setting the phase difference between the light beams, it is common to use a curved waveguide as at least one of the arm waveguides and change a geometric length along which the light beam propagates, so that an optical path length difference between the light beams from the respective arm waveguides is changed to thereby set the phase difference between the light beams. However, the optical path length difference between the light beams propagating through the curved waveguides is affected not only by the geometric lengths of the curved waveguides but also by the state of light confinement and an offset of a part connecting the curved waveguides. Therefore, there has been a problem in this method that the optical splitting ratio is unstable such that the light splitting ratio becomes wavelength dependent and polarization dependent.
In order to solve this problem, Patent Literature 1 discloses an optical splitter circuit using a tapered waveguide having a tapered structure instead of a curved waveguide as an arm waveguide. In this optical splitter circuit, a width-decreasing tapered waveguide, in which the width thereof gradually decreases, is used for one of the two arm waveguides, and a width-expanding tapered waveguide, in which the width thereof gradually expands, is used for the other. The optical path length of the light beam propagating through the tapered waveguide changes according to the effective refractive index determined from a taper angle and a width of the tapered waveguide. Thus, in this optical splitter circuit, the optical path length difference between the light beams propagating through the two arm waveguides can be adjusted by changing the effective refractive index of the arm waveguide without changing the geometric length along which the light beam propagates by means of a curved waveguide.